Yekeen

It was quite easy. You don't need to remove it from the wall. There are 2 screws on the front, hidden behind plastic plugs. I removed the plugs with sticky tape, and opened up the front part of the enclosure. Then the circuit board is attached to the rear part of the enclosure with 4 screws. The cable can be unplugged from the circuit board. The battery was a very tight fit to get out and in. I suspect that the battery holder that is attached to the circuit board might have been designed for a CR1220 instead of a CR1225. The only difference is that the CR1220 is 2mm thick while the CR1225 is 2.5mm thick. I got a CR1225 from Builders Warehouse for R30. Makro and Chamberlain only had CR1220's.

I reckon that would work just fine as you will only be charging 20Ah per battery and all 4 is well below the 150Ah max, so you can probably do one after the other without it getting too hot. It might just take a little longer as it's only 8A max current, but at top of charge the battery doesn't accept a lot of current anyway.